1. Field of the Invention
The present invention relates to the measurement of the distribution of thermal and/or mechanical stress over an area and, more particularly, to a mechanical/thermal stress transducer consisting of a piezo-pyroelectric material in electrical communication with a electroluminescent material, wherein an electrode positioned between the piezo-pyroelectric material and the electroluminescent material allows for electrical communication at disjunct intervals so that the luminous intensity of light emitted by the electroluminescent material at any point is related to the mechanical/thermal stress acting at that point on the transducer and distinct from stresses acting on adjacent points.
There are two types of materials which are useful for direct conversion of thermomechanical energy into electrical energy. Both of these materials will undergo a change in polarization when subjected to external stresses. One type consists of crystalline material in which the crystal structure possesses no center of symmetry. These materials have no net polarization when unstrained, but they respond to external stresses with internal strains which cause a separation of the center of positive charge from that of negative charge, thereby generating a dipole moment. When the external stresses are mechanical, such materials are properly termed piezoelectric. When the external stresses are thermal, such materials are properly termed pyroelectric. In general, a material which is piezoelectric will also be pyroelectric. An exception may occur when either type of response is vanishingly small.
The second type consists of material which has a spontaneous and reversible net polarization even in the absence of sustained external stresses. Such materials are properly termed ferroelectric. In response to external stresses, a ferroelectric material will change its net polarization. The difference between the two classes of material is that the ferroelectric material has a net polarization in the absence of external stresses whereas the piezoelectric or pyroelectric material is unpolarized in the absence of external stresses. As indicated by their ability to support a potential difference without an applied voltage, both classes are insulating materials under ordinary conditions.
Those experienced in the art refer to both the ferroelectric class of materials and the piezo- or pyroelectric class proper as "piezoelectric" or "pyroelectric" based on whether the primary stress effecting the change in polarization is mechanical or thermal, respectively. Since the response of either type of material is useful in the construction of transducers for thermomechanical stress and the response of the materials occurs under either mechanical or thermal stress, the term "piezo-pyroelectric" will be used hereafter to refer to either type of response. The abbreviation PPE will be used hereafter to refer to a piezo-pyroelectric material including ferroelectric materials.
Typical examples of PPE materials proper include quartz and tourmaline crystals. The ceramics lead zirconate-titanate (PZT) and barium titanate are typical of the ferroelectric materials. Another important type of ferroelectric material is the polymer ferroelectric, typified by poly(vinylidene fluoride), sometimes referred to as PVDF or PVF.sub.2. Ferroelectric sheets consisting of minute crystals of piezoelectric ceramics embedded in a polymer have also been employed. Some relevant physical properties of typical PPE materials are listed in Table 1.
Electrical current is carried in a semiconductor either by electrons or by electron vacancies which are referred to as holes by those skilled in the art. A semiconductor diode is a device consisting of two or more semiconductor layers, at least one of which is a hole conductor. A light emitting diode, commonly abbreviated as LED, is a semiconductor diode which converts electrical energy into electromagnetic radiation at visible and near-infrared wavelengths.
Most of the LEDs currently in use are composed of inorganic crystals. Typical examples are germanium, gallium arsenide, and zinc sulfide. Such materials do not lend themselves to construction into large area devices. Recently, organic polymers such as poly(phenylene vinylene) and poly(cyanoterphthalylidene) have been reported to have been used in the construction of LEDs (Vincent et al., Thin Film Solids, 1982; Bradley, J. Phys. D, 1987; Nohara et al., Chem.
TABLE 1 ______________________________________ Typical PPE material properties. Material Density Resistivity d.sub.33 p.sub.31 ______________________________________ Quartz 2.65 .sup. 10.sup.12 2.3 -- PZT 7.55 10.sup.6 245 275 PVDF 1.78 10.sup.8 20 40 ______________________________________ Units: Density, 10.sup.3 kilograms per (meter).sup.3 (Mg/m.sup.3) Resistivity, ohmmeters (.OMEGA.m) d.sub.33 , 10.sup.-12 coulomb per newton (pC/N) p.sub.3 10.sup.-6 coulomb per (meter).sup.2 per kelvin (.mu.C/m.sup.2 /K)
Lett., 1990; Braun and Heegar, Appl. Phys. Lett., 1991; Grem et al., Adv. Mater., 1992). These polymer LEDs may consist of a single layer of semiconducting material as just described or of several layers which transport only electrons, transport only holes, or are electroluminescent (Tang and vanSlyke, J. Appl. Phys., 1987; Adachi et al., Japan J. Appl. Phys., 1988a,b; Friend et al., Phys. World, 1992). The polymer diodes are often produced by methods which are consistent with production of large area devices. It is also fairly easy to change the material with ligands or dopants, a fact which leads to ready variations of photon energy and threshold voltage.
Recent advances have also identified transparent polymers which can be used as electrodes (Gustafsson et al., Nature, 1992). Previously, the transparent electrodes which permitted photon emission were composed of brittle materials. The new polymer electrodes allow the construction of flexible LEDs.
The drawbacks of existing transducers for measuring thermomechanical stress distributions fall into two categories. In the first category of drawback is their susceptibility to interference caused by electromagnetic fields in the environment in which they operate. The second category of drawback is that existing transducers are essentially devices for measurement of thermomechanical stresses at a single point or averaged over an area.
An exception to the second drawback is exemplified by the transducer in "night vision goggles" which consist of a sheet of pyroelectric material coupled to many individual solid state crystalline LEDs. The drawback of this device is that each LED must be individually mated with the pyroelectric sheet, which results in a device which is very uneconomical to construct.
2. Description of the Related Art
U.S. Pat. No. 4,748,366 discloses novel uses of piezoelectric materials for creating optical effects and describes a laminate combining a thin piezoelectric sheet with a thin sheet of an optical device. But these devices differ from the present invention in two fundamental ways. First, no method of separating the electrical output of one part of the piezoelectric sheet from another or the luminescent function of one part of the LED sheet from another is described, whereas the essence of the present invention is the separation of the response of one area from another. Second, the devices produce flashes when energized that are binary in their function, being either on or off. In contrast, the present invention specifically relies on the continuously variable nature of the light output of the LED sheet as the electrical output of the piezoelectric component changes.
U.S. Pat. No. 4,991,150 discloses an electroluminescent optical fiber shock sensor which specifically uses the continuously variable nature of the light output from an electroluminescent material. However, this device can measure only a single value of stress at a time. This value will be some average of the stress distribution acting over the entire surface of the transducer. No consideration is given to multiple stress measurements over a surface.
U.S. Pat. No. 5,193,911 discloses a thermal detector device which converts a spatial distribution of thermal energy into a multiplicity of electrical signals at each of a matrix of electrical terminals. This device illustrates the extreme difficulty of designing a thermal imaging detector from the paradigm of individual detectors located at very many points. Furthermore, the device would not operate well as a sensor for mechanical stresses because of its fragility. It consists of thin elements, including narrow strips, supported by posts. When subjected to many mechanical stress distributions of interest, deformations of these thin elements in response to inertial forces would cause the device to crack and/or tear before any useful signal could be obtained.